1. Field of the Invention
This invention relates to an objective optical system for endoscopes.
2. Description of Related Art
A video scope loaded with a solid-state image sensor at the distal end of the inserting section of the endoscope is progressing in a sleek design of the endoscope due to compactness of the solid-state image sensor. The video scope with an inserting section outside diameter of about 5 mm, having a channel for treatment and a distal end bending mechanism, has already been popularized. However, a video scope with an inserting section outside diameter of about 3 mm is being still developed because of its high technical hurdles. In such an extremely fine video scope, a sectional area allowable for the solid-state image sensor or the objective optical system is limited to about 1 mm2 or less. In the structure that the image pickup surface of the solid-state image sensor is provided perpendicular to an inserting axis (which is hereinafter referred to as “image pickup surface perpendicular placement”), it is very difficult to place circuit wiring and terminals adjacent to the outside of an effective image pickup section because of the size of the sectional area, and the mounting of the solid-state image sensor offers a great problem.
In contrast to this, the structure that the image pickup surface of the solid-state image sensor is provided nearly parallel to the inserting axis (which is hereinafter referred to as “image pickup surface parallel placement”) does not increase the sectional area even when the circuit wiring and terminals are provided adjacent to the outside of the effective image pickup section, and thus this is favorable for the extremely fine video scope. However, in the image pickup surface parallel placement, a prism and mirror bending the optical axis toward the solid-state image sensor are absolutely essential and a severe restriction is imposed on the objective optical system, together with a reduction of the sectional area of the objective optical system itself.
For the image pickup surface parallel placement of the solid-state image sensor and the compact design of the objective optical system, conventional techniques disclosed in Japanese Patent Kokai Nos. Sho 61-254917, Hei 9-262207, Hei 11-352413, Sho 60-26918, Hei 10-239594, Hei-5-341185, Hei 7-318799, and Hei 9-325285 are known.
The technique disclosed in each of Kokai Nos. Sho 61-254917, Hei 9-262207, and Hei 11-352413 refers to a prism for the image pickup surface parallel placement and a solid-state image sensor unit structure. The technique disclosed in each of Kokai Nos. Sho 60-26918 and Hei 10-239594 has reference to a relatively compact design in the objective optical system for the image pickup surface parallel placement. The technique disclosed in each of Kokai Nos. Hei 5-341185, Hei 7-318799, and Hei 9-325285 is related to the simplification and compact design of the objective optical system, not for the image pickup surface parallel placement.